Thank you for your reply!
In the meantime I did some additional testing after playing around in my BIOS. I will put my findings here, maybe they can help you.
I have put the VCORE setting in the BIOS to 'normal' and next I could change the VCORE OFFSET setting. I have changed it into a negative offset value of -0,080V. By doing this I could reduce the voltage during load (and also idle of course) and hence reduce the temperatures with around 10°C. That means that the VID voltage, measured by HWMonitor during the Intel Extreme Tuning Tool stress test, was reduced from 1,269V to 1,189V, and the temperatures went from max 73°C to max 63°C. Also the temperatures during the Prime95 small FFTs test went down a bit, from around 100°C to beginning 90's°C. The voltage during this test was 1,244V instead of 1,3+V.
The idle temperatures remained more or less the same, but the idle voltage decreased as well till 0,647V.
I'm actually not sure if my pc is running stable enough right now with these changes: therefor I should do a long (stress) test. I'm also not sure if this 'undervolting' is good for my system. And I don't think that this is default behavior that one must do in order to keep his/her cpu cold enough.
Besides this is 90°C during prime95 still a very high temperature I think.
Many thanks for your help!